The Wideband Code-Division Multiple Access (WCDMA) standard has been widely adopted in several third generation (3G) mobile communication systems. One major design challenge in the WCDMA transmitter, both for a mobile terminal and a workstation, is to improve the linearity and efficiency of the power amplifier (PA). This can be due to the non-constant envelope modulation and the multi-code scheme used in WCDMA. Nonlinear PA causes spectrum regrowth which results in significant adjacent channel interference (ACI). At present, the state-of-art linear PAs for the wideband applications provide about −40 dB adjacent channel power reduction (ACPR) at 5 MHz and −50 dB at 10 MHz, which fails to fulfill the 3G requirement on the output spectral mask. However, this gap can hardly be solved by PA back-off which will cause severe losses in power efficiency.
The third generation wireless systems place much more difficult linearity and efficiency requirements for the RF front-end. The linearity constraint is due to tighter output spectral mask specification, higher signal envelope variations (linear modulation), and, in the case of the PA, the need to keep the operation level near the compression point in order to achieve a high enough efficiency. In addition, when multi-code transmission is applied, more backoff is needed, causing a loss of efficiency. Linearization techniques are considered as one possible solution to overcome the tightened spectral mask requirements with acceptable amplifier efficiency.
Linearization techniques can be divided into four main categories: (1) feedforward, (2) feedback, (3) envelope elimination and restoration, and (4) predistortion. Each of these have a set of variants providing different implementation complexity, adjacent channel interference (ACI) improvements, and bandwidth/convergence rates.
The first three categories are suited for analog implementation. Feedforward can, in theory, completely eliminate the inter-modulation distortion, but the key problem of this scheme is the need of perfect gain and phase match between the two signal paths. The complexity of this scheme is quite large and the total efficiency is drained due to losses in the main path delay, the couplers and the auxiliary amplifier. Among the various feedback techniques, Cartesian feedback is most prominent and thoroughly studied. It has been proven to work for wideband applications. Polar modulation feedback is most suitable for narrowband systems. The power efficiency of these techniques is low for low input levels. Moreover, the complexity of these schemes is also quite high.
In the envelope elimination and restoration scheme, a modulated intermediate frequency (IF) signal is split into its polar components. The constant-envelope IF signal is translated to RF with a mixer and amplified to a level forcing the power amplifier to saturate. The envelope is restored by modulating the supply voltage to the power amplifier with the detected IF envelope. For more information, please see L. Sundstrom, “Digital RF Power Amplifier Linearisers—Analysis and Design,” Dissertation for the degree of Ph.D, LUTEDX/(TETE-1013)/1–150(1995), Lund university, Sweden, August 1995, which is hereby incorporated by reference in its entirety.
Predistortion can be realized at baseband by the DSP techniques or at RF with nonlinear devices. Digital baseband solution is usually preferred, since it is better suited for tracking any possible change in PA parameters. Mapping predistortion has been proposed, using a huge two-dimensional table. The more memory efficient scheme is the complex gain predistortion which has a one-dimensional table and can compensate phase invariant nonlinearities. Adaptive algorithm is frequently used for tracking the variations of the PA parameters. It requires large computing power and a dedicated feedback loop. Available research shows that it is suited for narrowband systems only.
Accordingly, what is needed in the art is a WCDMA transceiver which employs linearization techniques that overcome the limitations of the prior art.